fsync(2) and on-disk write-back cache

Jeremy Chadwick freebsd at jdc.parodius.com
Tue Aug 31 16:08:43 UTC 2010

On Mon, Aug 30, 2010 at 06:58:42PM -0400, Martin Cracauer wrote:
> I always assumed the answer to this question is "of course":
> When doing an fsync (waiting for the commit), do we actually tell the
> disk to flush the on-disk write-back cache (if that is in use) to the
> platters?
> I just went down some code paths in both FreeBSD and Linux and in both
> cases the paths for fsync quickly disappear in the generic
> block-by-block flushing code that is also used for regular (non-fsync)
> flushing.  I didn't see anything aware of the on-disk cache.

I don't have an authoritative answer to your question, but this thread
seems to imply there's a relation between fsync() and an intentional
disk flush (BIO_FLUSH).  I'm sure when BIO_FLUSH is called depends on
the filesystem as well.

Funny timing this topic, given a post over in -stable:


| Jeremy Chadwick                                   jdc at parodius.com |
| Parodius Networking                       http://www.parodius.com/ |
| UNIX Systems Administrator                  Mountain View, CA, USA |
| Making life hard for others since 1977.              PGP: 4BD6C0CB |

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